In Episode 3 of Season 3 ("Fireflies"), Lydia heads out to the store at night, but
somehow ends up instead at a public pool in a park.

In a dreamlike sequence, she sees a body floating face down in
the pool, but when
she turns it over, she discovers, to her relief, that it is just a CPR dummy.

But then Lydia she sees blood dripping from above her. Looking up, she sees a
real corpse,
a young man with his throat slashed and drenched in blood, propped up in the lifeguard's
chair, above the diving board.

Later, Stiles shows up and talks to the confused girl. He phones Scott, who asks him to take a
closer look at the body. When he does, Stiles notices a purity ring on the dead man's finger.

This scene was actually filmed at the swimming pool at

Birmingham High School
at 17000 Haynes St, Van Nuys, CAThe pool is near the center of campus, northwest of the stadium.
The body/dummy Lydia sees was floating in the southeast corner of the pool,